A rocket engine is typically provided with a reservoir of cryogenic fuel and a reservoir of cryogenic oxidizer for the fuel. The cryogenic fuel and the cryogenic oxygen are pumped in liquid form to the rocket chamber where the fuel and oxidizer are burned to produce thrust for the rocket.
Turboprops are used to pump these fuels. Each turbopump has a turbine and a pump. The turbine has a rotor assembly which is connected to the pump. A flowpath for hot, pressurized working medium gases extends axially through the rotor assembly. The rotor assembly is driven by these gases about its axis of rotation.
Typically, the rotor assembly has a rotor disk and one or more arrays of rotor blades. The rotor blades extend outwardly into the working medium flowpath. The rotor blades engage the outer periphery or rim region of the rotor disk. The rim region of the rotor disk is adapted by axially oriented slots to receive the rotor blades.
The working medium gases exert a tangential force and an axial force on the blades as the gases are flowed through the rotor assembly. The axial force on the rotor blades urges the rotor blades axially rearwardly and out of the blade retention slots. Lock means are provided to lock the blades against axial movement. These locks add to the rotational mass of the rotor assembly and must be carried by the rotor disk.
The tangential force exerted by the working medium gases drives the rotor assembly about its axis of rotation. As the rotor assembly rotates, the rim region of the disk between the slots can only exert a radial force on the blades to retain the blades against outward movement. Because the blade attachment slots interrupt the circumferential continuity of this portion of the rim region, this part of the rim region is unable to generate circumferential ("hoop") stresses to resist outward movement of the blades. This circumferentially discontinuous region is referred to as the "dead rim" of the disk. The circumferentially continuous part of the rim region inwardly of the dead rim region resists the radial rotational loads exerted by the dead rim with hoop stresses. This part of the rim region is referred to as the live rim region of the disk because of its ability to sustain hoop stresses.
The radial forces or rotational loads in the dead rim are aggravated by stress concentrations associated with the fillets used to form the blade retention slot or fillets in other parts of the disk.
Additional stresses in the dead rim resulting from axial temperature gradients (thermal stresses) may cause, in combination with the rotational stresses, unacceptably high stresses in the dead rim.
An example of a construction used to retain rotor blades in gas turbine engines is shown in U.S. Pat. No. 4,444,544 issued to Rowley entitled "Locking Of Rotor Blades On A Rotor Disk". In this construction, the rotor blade has a half-groove formed by an extension from the rotor blade facing a corresponding half-groove in the rotor disk. A lock pin is inserted in the cooperating grooves to lock the rotor blade against axial movement. The radial load exerted by the rotor blade on the rotor disk is increased by the mass of the groove on the rotor blade and the mass of the blade lock. In high speed rotation, these masses may cause additional stresses which may in some constructions be unacceptable. No effort is made to shield the dead rim region of the disk.
Another example of a construction providing axial retention is shown in French Patent 1,037,572 issued to Haworth and Petrie. A projection from the rotor blade has a groove. A cooperating groove is formed in an extension from the live rim region of the rotor disk. A lock pin is inserted in these grooves to restrain the blade against axial movement. No shielding is provided to the dead rim.
An example of a construction for axially retaining the rotor blades and shielding the region of the rotor blades is shown in U.S. Pat. No. 3,137,478 entitled "Cover Plate Assembly For Sealing Spaces Between Turbine Buckets" issued to Farrell. The cover plate assembly engages the blade retention slot and shields the side of the turbine blades and the dead rim from hot working medium gases. The rotational loads of the cover plate are transmitted to the disk through the dead rim region of the disk which increases stresses in the dead rim.
In the Rowley, Haworth and Farrell constructions, the mass of the extension on the rotor blade and the mass of the locking device are radial loads which are transmitted to the live rim of the rotor disk through the dead rim region of the rotor disk. These radial loads at the side of the disk combined with the radial loads of the rotor blade and thermal stresses resulting from contact with the hot working medium gases or with cooling gases adjacent the rotor disk may cause significant stresses in the dead rim region of the disk or at other locations in the rim region. The problem is complicated by the stress concentrations which result from fillets of the blade retention slot.
U.S. Pat. No. 4,021,138 issued to Scalzo et al, entitled "Rotor Disk, Blade And Seal Blade Assembly For Cooled Turbine Rotor Blades" forms an enclosed cooling chamber on a rotor disk. The seal plate which forms one side of the chamber is trapped radially against an inwardly facing local cam surface which is a circumferentially interrupted projection from the disk.
The above notwithstanding, scientists and engineers working under the direction of Applicant's Assignee have sought to decrease thermal stresses and rotational stresses that occur in the dead rim region of the rotor disk with a rotor blade lock having few parts and low radial profile to minimize the mass of the lock.